Oct 15 2009
Contrary to expectations, structurally different molecules can display different solvent properties at an interface between air and water, researchers in Japan have discovered1. Tahei Tahara and colleagues from the RIKEN Advanced Science Institute in Wako showed that polarity at this interface cannot be defined simply, because it depends on the nature of the solute molecule at the interface. The finding could have significant consequences for chemistry at interfaces, since the polarity of a molecule's environment affects how it reacts with other molecules. Fields such as atmospheric science, where air/water interfaces abound, will be particularly affected.
The researchers made their discovery using an interface-selective spectroscopic technique that they developed earlier2. The spectra that the technique produces are of comparable quality to those of bulk solutions, enabling previously impossible comparisons between systems.
The researchers looked at the electronic spectra of five coumarin dyes at the interface between air and water; electronic spectra are essentially a graphical representation of a molecule's color. Coumarin dyes all share the same basic chemical structure and are used to probe the polarity of solvents because their spectra differ depending on the molecules' environment.
Tahara and colleagues found that the spectra of all five coumarin dyes at the air/water interface resembled a cross between the bulk spectra of coumarin in polar water and non-polar hexane. This is because the dye molecules were positioned partly in the polar water and partly in the non-polar air at the interface. However, the closeness of the spectra to either the spectrum in water or in hexane changed depending on the precise structure of each coumarin dye.
Previously it was thought that, in ordinary cases, molecules experience the same polarity—the average of that of polar water and non-polar air. The spectra Tahara and colleagues measured, however, showed that even molecules having similar structures experience substantially different polarity at the air/water interface.
The researchers found that the different molecules were positioned at slightly different angles at the interface of air and water so have different sections of their structures submerged and are, consequently, in quantitatively different surroundings.
“This work showed that, even at the same air/water interface, the interaction between the solute and solvent is significantly varied,” says Tahara. This means the molecules experience different environments at the interface, similar to being in different solvents from a view point of the stabilization energy. “This fundamental understanding of molecular behavior will be very important when people consider chemical reactivity at liquid interfaces.”